1. Field of the Invention
This invention relates generally to an apparatus for detecting power arcing faults, and more specifically, to such an apparatus for detecting power arcing faults on low-voltage spot networks.
2. Description of the Prior Art
Low-voltage electrical power networks consist of interlaced loops or grid systems. These systems are always supplied with electrical energy by two or more power sources, so that the loss of any one source does not result in a power interruption. Such systems provide the highest level of reliability possible with conventional power distribution and are normally used to serve high-density load areas. Primary applications include central or downtown city areas, large buildings, shopping centers, and some industrial plants. These network systems are of either the grid type or spot type with three-phase three-wire or three-phase four-wire service at 208Y/120V or 480Y/277V.
A grid-type network includes a plurality of sources and loads arranged in a grid pattern. Each source has a dedicated feeder supplying the network via a high voltage switch, a network transformer, and a network protector. The network protector comprises a circuit breaker and protective relay, for isolating the transformer from the network bus when a fault occurs in the transformer or the primary circuit feeding it.
Large concentrated load areas, such as commercial buildings and shopping centers, are frequently served by spot networks, i.e., two or more network units fed by two or more primary feeders. Typically, the spot network primaries are tapped from non-dedicated feeders. The feeders are connected to the three-phase transformer primary, and the secondary thereof is connected to the network via a network protector and fuses. The typical low-voltage spot network installation is operated at a nominal voltage of 480 V (line-to-line voltage grounded wye configuration) and protected by fuses typically rated at 4,000 to 6,000A. As in the grid-type network, the network protector of a spot network isolates the transformer from the bus in the event of a fault in the transformer or the primary circuit feeding it. This arrangement ensures high reliability since primary faults are isolated and the network is carried by other feeders connected to it. The network protector is designed to open when the protective relay associated with it detects a fault on the primary feeder that would cause power flow from the network into the feeder, that is, reverse power flow. The network protector is not, however, designed to open for faults on the network itself. Also, the network protector is designed to open when the primary feeder is disconnected from its source of supply and magnetizing current flows from the secondary network into the network transformer. Typical network installations do not contain phase overcurrent or ground overcurrent protective relays or any form of overcurrent protection for the network, except for the fuses.
It is difficult to detect power arcing faults on the 480 V networks because the arc voltage limits the fault current to several thousand amperes or less, which is below full load current. As a result, the fuses do not open and extensive arc-burning damage may occur to the equipment in a spot network. One apparatus for detecting such power arcing faults is described in copending U.S. patent application Ser. No. 610,540 (filed May 15, 1984) This prior art patent application describes, in a general way, a device for detecting power arcs on a low-voltage spot network. The present invention represents an improvement to the prior art device by including means for discriminating between high-frequency harmonics caused by a power arcing fault and sporadic or steady-state high-frequency harmonics unrelated to a power arcing fault.